Epithelial tissues, being involved in the net transport of the solutes and water, are known to possess cells whose apical and basolateral membranes are physiologically different with regard to their ionic permeability and their sensitivity to various drugs and hormones. The apical membranes of tight epithelia, like the frog skin, are highly permeable to Na, and they play an important role in the mechanism that regulates active transepithelial Na transport. These membranes show a nonlinear dependence of Na transport on the apical solution sodium concentration and appear to electrically rectify the flow of Na. The mechanisms of Na entry and the mechanisms of control are not well understood. In this regard, fluctuation analysis of the apical membranes are to be done to assess the voltage and Na concentration dependency of the Na channels at the apical membrane of the epithelium of frog skin from measurements of the amiloride specific Na channel noise fluctuations. The method permits the density of channels and the Na current per channel to be measured and this taken together with parallel studies utilizing intracellular microelectrode techniques should permit analysis of the Na channel conductance and those factors that change its properties.